There is a known endoscope that has a wide viewing angle, i.e., 180° or more, and that allows simultaneous observation of forward, side, and even backward fields of view (see, for example, patent literature PTL 1 below). For example, if an endoscope having a viewing angle of 180° or more is used in the large intestine, where numerous folds are found, the back side of a fold can be observed without changing the orientation of the distal end of the endoscope by a large amount, which allows the user to manipulate the endoscope more easily and to find an affected area more reliably. The endoscope in PTL 1 has, at the distal end thereof, an illumination device including two light guides and illuminates forward and side fields of view with different light guides to illuminate a wide field of view in its entirety.
Endoscopes have various components concentrated at the tips thereof, including image-capturing optical systems for capturing images of subjects, channels for surgical instruments, nozzles for lens cleaning, and mechanisms for bending sections thereof. Accordingly, there is a need for an illumination device that can be installed in a thin endoscope within the minimum possible space in the radial direction. The illumination device in PTL 1 includes a plurality of light guides arranged side-by-side in the radial direction in the surrounding part of the image-capturing optical system, and the distal end of the light guide for side illumination is bent in the radial direction of the endoscope to achieve side illumination. This requires a large installation space in the radial direction of the endoscope and thus results in a large increase in the tip diameter of the endoscope.